A prosthetic foot must provide stable support to the user under a variety of conditions. Such conditions include a variable stride and a range of different activities. In particular, a prosthetic foot has long been sought that can provide stable support for a user who is walking on an ever changing terrain, such as that encountered in normal daily activity. To achieve this objective, a prosthetic foot would ideally provide a range of motion in a medial-lateral direction. It is also desirable that the prosthetic foot has energy storage capabilities to provide a more normal gait.
Dynamic response prosthetic feet are preferred for active amputees. The energy storage capabilities of the feet give them a spring-like functionality, which improves the feel and overall function of the prostheses. Two widely used types of prosthetic feet are high profile dynamic feet and low profile multiaxial feet.
High profile dynamic feet consist of a long L-shaped piece of material attached to a base plate. The L-shaped piece of material may be alternatively referred to as a frame. Typically, the frame is elastic and therefore provides some energy storage capability. Generally, the frame is a composite, such as a carbon fiber laminate or a polymeric material. At present, all high profile dynamic feet have a rectangular cross section relative to the frame and therefore movement of the footplate is typically limited. Such high profile dynamic feet have advantages because of their high-energy storage capability. High-energy storage occurs in the frame of the prosthesis. High profile dynamic feet have the longest frame, and thus act as the biggest springs and, accordingly, store the most energy. However, high profile dynamic feet also present some drawbacks. Principally, the high profile dynamic feet have no ankle motion and therefore are not capable to conforming to a changing terrain. The foot portion of a high profile dynamic foot stays in the same position relative to the frame regardless of whether the amputee is walking on an incline, walking on uneven terrain or moving in a side-to-side direction.
The multiaxial dynamic feet of the prior art attempt to simulate motion of the ankle and are generally considered more stable than high profile dynamic feet. A disadvantage of the multiaxial dynamic foot is that generally it is a low profile prosthesis. By comparison to the high profile dynamic feet, low profile multiaxial dynamic feet can only store energy in their keel, which is a much smaller frame and, thus, a much smaller spring. Accordingly, there is correspondingly less energy storage. Therefore, with the prior art devices, there presently is a tradeoff between increased stability and improved energy storage capability.
Typically, the multiaxial feet of the prior art possess an axis of rotation through the ankle joint that lies transverse to the normal anterior-posterior alignment of the foot. Subsequently, such multiaxial prosthetic feet, although typically providing a range of rotation in an anterior-posterior direction, have limited freedom to move in a medial-lateral direction. Subsequently, the prior art multiaxial feet typically only allow a small amount of medial-lateral tilt and do not allow true rotation in the medial-lateral direction. Tilt is distinguishable from true rotation in that tilt may occur along any of a multitude of axes, whereas rotation occurs about one axis. Tilt may also be described as wobble. Typically, in the prior art devices, the amount of medial-lateral tilt is a consequence of some looseness in the ankle joint. This looseness is generally accomplished through the use of an elastomeric member in the ankle joint, which member can then compress to a limited degree, thus accommodating medial-lateral tilt. However, it can be difficult to control the tilting motion. Generally, the prior art devices do not possess an axis of rotation through an ankle joint where the axis of rotation lies in the anterior-posterior direction. Furthermore, elastomeric member tends to wear out.
Some of the prior art prosthetic feet provide a range of motion in an anterior-posterior direction. Providing this range of motion is accomplished, for example, by providing a flexible foot that includes an ankle member that flexes in the anterior-posterior direction. Another prior art device provides anterior-posterior motion by using a very high modulus material that permits limited deformation under a high load. This high modulus elastic material is typically positioned between the foot and the frame. As the load on the frame changes during the normal transfer of weight that occurs during walking, the high modulus elastic material flexes to a limited extent. Still another prior device uses an o-ring positioned at the end of the frame where the frame connects to the foot. This o-ring is typically made of a high modulus material and will deform to a limited extent as weight is transferred during a normal stride.
Typical among the devices that rely on a high modulus elastic material for flexibility, the range of motion is necessarily limited. If the material forming at least part of the connection between the frame and the foot has too low a modulus, then control over the foot during normal walking will be compromised. Some of the prior art suggests that a limited degree of medial-lateral movement will occur as a result of the compression of the high modulus elastomeric material positioned between the frame and the foot. Such movement has been described in the art as a slight rocking or a slight tilting motion. The prior art further teaches that although some medial-lateral rocking motion can be accomplished, generally, medial-lateral movement is resisted.
Still another prior art device provides an elastomeric bushing about a heel ankle connector pin. As with the other prior art devices already described, this bushing material is a high modulus elastomer. Accordingly, some compression of the elastomer may take place during the normal weight transfer accompanying walking and result in a small amount of medial-lateral tilt or wobble. As provided above, prostheses that rely on a high modulus elastic material for flexibility tend to have a problem with durability because they wear out with repeated loading and unloading.
Lack of a true hinge allowing rotation in the medial-lateral direction is a disadvantage. True control of movement in a medial-lateral direction about a unitary axis is difficult to achieve in the prior art devices. Any medial-lateral movement in these devices is limited to a tilting movement. This tilting movement can occur along any of an infinite number of axes. Because it is not possible to control every possible axis along which the prior art foot may move, medial-lateral directional control can be difficult to achieve and, therefore, medial-lateral movement is typically constrained. Not surprisingly, the prior art devices limit a full range of medial-lateral movement. For example, in many prior art multiaxial feet, movement of the foot occurs through compression of an elastomeric pad positioned in the ankle region of the foot. Thus in order to provide movement of the foot in a medial-lateral direction the entire elastomeric pad must be of a modulus that affects movement in all directions. In such a multiaxial foot, there is no independent control over movement in a singular direction or line of action.
It would therefore be an advantage to have a prosthetic foot that offered the stability advantages of a multiaxial dynamic foot with the energy storage capabilities of a high profile dynamic foot. It would be an even further advantage to have a prosthetic foot that allows true medial-lateral rotation. It would be at an even further advantage to have an adjustable prosthetic foot that would allow the manufacturer or wearer to select a range of medial-lateral rotation best suited to a wearer's needs.
It would be an even further advantage to have a high profile multiaxial prosthetic foot that could allow free rotation about an axis that lies in the anterior-posterior direction.